The present invention relates to semiconductor lasers, and more particularly to a buried heterostructure semiconductor laser capable of single frequency operation and suitable for use in optical fiber communication or the like.
Conventional buried semiconductor lasers, though having such advantages as low lasing threshold current and stable fundamental transverse mode operation, involve a problem when they are to be used in long-distance large-capacity communication systems because of the heavy temperature dependence of their lasing wavelength and their great spectral width. To solve this problem, there has been an attempt to provide such a buried semiconductor laser with a resonator having an internal periodic structure instead of a cleavage facet. (See K. Kobayashi et al., "CW Operation of 1.5.about.1.6 m Wavelength GaInAsp/InP Buried-Heterostructure Integrated Twin-Guide Laser with Distilled Bragg Reflector", ELECTRONICS LETTERS, Vol. 17, No. 11 (May 28, 1981), pp. HH-368).
However, since only p-InP and n-InP layers bury an isolated reverse mesa stripe, the proposed structure has these disadvantages; (1) it is difficult to precisly control the burying; (2) the mesa stripe is susceptible to damage during the burying process, and it is accordingly difficult to obtain an efficient product, and (3) because the surface thus formed is not sufficiently flat, it is not suitable for an upside-down mounting, which is desirable for high-temperature operation. Furthermore, the structure in which integrated twin guides are separated by a separation layer makes it considerably difficult to achieve satisfactory coupling of an active layer and a guide layer constituting the twin guide.